Transdermal drug delivery, as the term is used generally, refers to permeation of the stratum corneum, the tough outer barrier of the skin, by a pharmaceutically active molecule. The stratum corneum, the thin (approximately 10-20 μm) outer layer of the epidermis, is dead tissue containing both multilamellar lipid barriers, and tough protein-based structures.
The epidermis, directly beneath the stratum corneum, also behaves as a lipid barrier. The dermis, directly beneath the epidermis, is permeable to many types of solutes. In the administration of a drug by topical application to skin, lipid-soluble drug molecules dissolve into and diffuse through the skin's multilamellar lipid bilayer membranes along a concentration gradient by virtue of the drug molecules solubility in the lipid bilayer. Transdermal drug delivery may be targeted to a tissue directly beneath the skin, or to capillaries for systemic distribution within the body by the circulation of blood.
The term transdermal drug delivery usually excludes hypodermic injection, long-term needle placement for infusion pumps, and other needles, which penetrate the skin's stratum corneum. Thus, transdermal drug delivery is generally regarded as minimally invasive. However, the low rate of transport of therapeutic molecules through the stratum corneum remains a common clinical problem.
It is an object of the present invention to provide a device and method that reduces or wholly overcomes some or all of the difficulties inherent in prior known devices and methods. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.